Power device with synchronization of plural actuators

ABSTRACT

A power device having a plurality of hydraulic actuators, and means for synchronizing the speed of movement thereof, including a sensing member driven by each actuator, one of the actuators being a master and the others slaves, the sensing members of the slave actuators being interfitted with the sensing member of the master actuator, and upon variation in speed of any of the slave actuators relative to the master actuator, the consequent interengagement of the sensing members controls an electric circuit and increases or decreases flow of fluid to the respective slave actuators according to relative slower or faster movement thereof.

United States Patent Visser [4 1 Sept. 19, 1972 [54] POWER DEVICE WITH 7 3,381,833 5/1968 Gordon ..60/97 E X SYNCHRONIZATION OF PLURAL 2,312,213 2/1943 Ferris ..60/97 E X ACTUATQRS 2,380,973 8/1945 Kopp ..91/171 [72] Inventor: Peter J. Visser, Niles, Mich. Primary m Geoghegan [73] Assignee: Clark Equipment Company Attorney-Paul Gallagher I [22] Filed: June 25, 1970 57 ABSTRACT I [2]] Appl- N03 49,608 A power device having a plurality of hydraulic actuators, and means for synchronizing the speed of move- 52 US. Cl. ..91/412, 91/1, 91/171, there, including a Sensing member drive" by 60/97 E each actuator, one of the actuators being a master and 51 1m. 01 ..FlSb 11/22 the Others Slaves the sensing members 0f Slaw [58] Field of Search 9 4 2 1 171. 0/97 E 1113,1018 being interfitted with the sensing member of the master actuator, and upon variation in speed of l 56] Referehces Cited any of the slave actuators relative to the master actuator, the consequent interengagement of the sensing UNITED STATES PATENTS members controls an electric circuit and increases or decreases flow of fluid to, the respective slave actua- 4/1910 Coyxe '1 n X tors according to relative slower or faster movement 2,487,402 11/1949 Watson ..60/97 E thereof. 3,262,740 7/1966 Stockwell ..91/171 X 3,355,886 12/1967 Weisenbach ..91/412 X 13 Claims, 14 Drawing Figures PATENTEBsEP 19 I972 3,691,9 1 1 SHEET 1 [IF 6 FIG. I

MASTER llllllh llllllllllllnu FIG. 8

ATTORNEY PATENTEDSEP 19 I972 SHEET 2 BF 6 ATTORNEY PATENTEDSEP 19 1972 SHEEI 5 BF 6 INVENTOR PETER J. VISSER ATTORNEY PATENTEnszr 19 I972 3,691.91 1

' sum 6 or 6 FIG. l3

INVENTOR PETER J. VISSER ATTORNEY POWER DEVICE WITH SYNCIIRONIZATION OF PLURAL ACTUATORS FIELD OF THE INVENTION The invention relates to power devices which are used most commonly as lifting devices, having a plurality of actuators, all of which must move at substantially the same speed, within very small limits of variation. Variation in the-speed of movement of the different actuators beyond those limits can cause serious damage, such as by twisting the actuators or the load being moved.

Devices of this general nature have been provided with synchronizing means which have reached various degrees of success, but the functioning of such power devices becomes more demanding as the loads tobe moved, orthe jobs to be performed, become more refined. In the case of hydraulically operated actuators,

flow dividers are often utilized, and these perform a function in synchronizing the movement of the actuators, but the various actuators are subject to factors independent of the flow dividers, which cause variation in the rate of movement of the various actuators, such factors being for example difference in cylinder leak, and drag factors.

The present apparatus is directly responsive to the actual variation in the speed of movement of the actuators, independently of other preventatively operating means such as flow dividers.

The apparatus can be used either for lifting, or for moving otherwise, such as pushing in a horizontal direction, as well as for synchronization of rotary motions.

OBJECTS OF THE INVENTION A broad object of the invention is to provide a power device having a plurality of actuators, and a novel arrangement for synchronizing the movement of the actuators.

Another broad object is to provide a power device of the character just referred to including means for synchronizing movements of the actuators in which the actuators may be more than two in number and can theoretically be of any number within the limits of working practicalities, in which all of the actuators are controlled for movement within the small limits referred to.

A still further object is to provide a power device having a plurality of actuators, incorporating novel sensing devices including elements individual to the actuators and movable directly as the actuators are moved, and simultaneously therewith, the sensing devices being arranged for interengaging upon variations in speed of movement of the different actuators, and operative for effecting change in speed of the different actuators according to such variations, whether of increased or decreased speeds in any of the actuators, for correcting such variations.

Still another object is to provide apparatus of the character just immediately referred to which is of hydraulic character, and wherein the means for controlling the speed of movement of the actuators includes a novel arrangement for controlling the flow of fluid to the different actuators, by increasing that flow to the actuators which move below a predetermined rate, and decreasing the flow to those actuators which exceed that predetermined rate of movement.

- tor, and an electric circuit and a hydraulic circuit are provided responsive to suchinterengagement for compensating for the variation in movement between the different actuators, whereby the movement of the master actuator determines the rate of movement of all of the actuators.

DESCRIPTION OF A PREFERRED EMBODIMENT In the drawings:

FIG. 1 is a face view of a power device including two actuators, and thus embodying the invention in the simplest form;

FIG. 2 is a perspective view of a known type of device, a van carrier, to which the invention is particularly adaptable, and including the components represented in FIG. 1;

FIG. 3 is a fragmentary detail view, semi-diagrammatic in nature, of the sensing elements and related elements associated with two actuators, and containing a reference line A-A not related to any other figure;

FIG. 4 is a view taken at line 4-4 of FIG. 3;

FIG. 5 is a diagram of the sensing element arrangement and four actuators;

FIG. 6 is a diagram of the electrical and hydraulic circuits utilized in the device;

FIG. 7 is a diagram of the hydraulic circuit including material additional to that of FIG. 6;

FIG. 8 is a fragmentary detail view, also semi-diagrammatic in nature, similar to FIG. 3 but showing a modified form thereof;

FIG. 9 is a diagrammatic view representing the invention applied to a 4-wheel vehicle and utilized in the steering thereof;

FIG. 10 is a diagrammatic view also applied to a vehicle and utilized in steering the vehicle, but utilizing only two sensing means as contrasted with four sensing means of FIG. 9;

FIG. 11 is a semi-diagrammatic view representing the invention applied to the fork of a lift truck;

FIG. 12 is a semi-diagrammatic view representing the invention applied to elements of an air craft;

FIG. 13 is a view representing the invention applied to a power device utilized in horizontal movement, as distinguished from lifting; and

FIG. 14 is a semi-diagrammatic, perspective view of.

the invention applied to rotated members.

FIG. 1 shows a power device embodying the invention in its simplest form, i.e., a single lifting member 24 including two actuators, while other views show more than two actuators, typically four, being made up of a pair of the lifting members shown in FIG. 1. The lifting member 24 of FIG. 1 includes a pair of hydraulic actuators or jacks 28 and a top cross piece 29.

Various elements and members of the structure appear repeatedly, and for convenience in referring to the relation therebetween, certain of them are individually identified with the subscripts, a, b, c and d.

FIG. 2 shows what is generally known as a van carrier, indicated at 31 and which in its general construction may be of desired type, such as shown in Bjorklund U.S. Pat. No. 3,l46,903, dated Sept. 1, 1964. The van carrier rides over and engages and lifts a van 33 or other corpus, and carries it to a different location and lowers it and leaves it there. The carrier includes a pair of lifting members 24, thus having four actuators, but there may be a lesser or greater number than that. It is essential that the actuators 28 all move at substantially the same rate, within very narrow limits, in order to prevent damage to the actuators or to the load, such as by canting the actuators or twisting the load.

With reference to FIG. 1, each actuator 28 includes a cylinder 30, and a piston 32 provided with a rack 34 meshing with a pinion 36 mounted by suitable means on the cylinder 30 on an axis 38. In the present instance the actuator 28a is designated the master actuator and as such determines the rate of movement of all of the actuators in a manner described below, and the actuator 28b is a slave actuator, which is regulated as to its rate of movement according to the rate of movement of the master actuator, and in the case where there are more than two actuators, all but one are slave actuators and are controlled in the manner of the actuator 28b.

Secured to the pinion 36a (FIG. 3) is a disc 40 forming a sensing member and including electrical contact elements 42b connected with slip rings 44. Connected with the pinion 36b is a snare, or flexible wire 45, such as a bowden wire or a speedometer cable which in turn is connected with another disc 46 concentric with the disc 40 and having a radial finger 48 extending between the contact elements 42a, 42b for engagement therewith in a manner described hereinbelow. The sensing disc 46 is rotated according to rotation of the pinion 36b in the same direction, or complementary direction, depending upon the positioning of the discs and the direction of extension of the line 45 and as viewed axially of the pinion and disc. For example as viewed in the plane of the line A-A of FIG. 3, the pinion 36b may rotate clockwise, and when it does so, the disc 46 rotates counterclockwise, as does the disc 40, but the specific directions of movement of these members may be as desired according to the particular application of the device.

The finger 48 is electrically connected with a slip ring 50. A plurality of brushes 52 are arranged for electrical contact engagement respectively with the slip rings and connected with these brushes are electrical conductors 54, 56, respectively leading to a battery or other electrical power terminal and electrically controlled valves 60 for controlling the actuators 28, in a control electric circuit as described more fully hereinbelow.

In the operation of the device of FIGS. 1, 3, 4, in the movement of the pistons 32, and for example in extension direction, it is of course desired that these pistons move at exactly the same rate. As the pistons so move, they rotate the pinions 36a counterclockwise and 36b clockwise and thus the sensing discs 40, 46 counterclockwise and so long as the pistons move at the same rate, the discs continue to move at the same rate. They are prepositioned so that the finger 48 (FIG. 4) is midway between the contacts 42a, 42b and so long as the pistons move at the same rate the finger remains midway between those contacts. However assuming there is a differential in the rate of movement of the pistons, the finger 48 will engage one or the other of the contacts 42a, 42b and complete a circuit as represented in FIG. 3 for correcting that differential. If the actuator 28b and subsequently the disc 46 should move faster, the finger 48 would engage the contact 42b and establish a circuit through the conductor 56b and actuate the valve 60b which would, in that case, reduce the flow of fluid to the actuator 28b, causing said actuator 28b to move slower. On the other hand, if the actuator 28b should move at a slower rate, the finger 48 (FIG. 4) would lag behind and engage the contact 42a and establish a circuit through the conductor 56a and the electrically operated valve 60a and in that case effect a decrease in flow of fluid to the actuator 28a. The spacing between the contacts 42 determines the fineness or coarseness of the control exercised. The specific arrangement of control of fluid to the actuators is described in detail hereinbelow.

FIG. 5 shows the sensing arrangement for four actuators, and as used in the van carrier 31 of FIG. 2. In the case of four actuators, the same two actuators 28a, 28b and related elements as included in FIG. 1 are included, and additional ones are identified by the same reference numerals with subscripts c and 11 respectively. Cables 45 drivingly connect the pinions 36 with the respective sensing members. The sensing disc 40a identified above is mounted on a gear 62 in mesh with another gear 64 having sensing discs 40c, 40d mounted thereon, the sensing discs 4011, 40c, 40d thereby all rotating at precisely the same rate. The sensing disc 46 of FIG. 3 is also included in FIG. 5 here individually identified as 46b, and the arrangement includes other sensing discs 46c, 46d associated with the sensing discs 40c, 40d. The sensing discs 46c, 46d are also provided with fingers 48 engageable with the contacts 42 as described above in the respective sensing members.

FIG. 6 shows the effective electrical circuit, utilized in the apparatus, and a portion of the hydraulic circuit, including the sensing discs 46b, 46c, 46d and a common conductor 66 leading to the fingers 48. The contacts 42 are connected in circuit with switch means 68 actuated by a manually operated valve 70 which controls the actuators for extending them or contracting them for correspondingly lifting or lowering the load. The diagram of FIG. 6 indicates the four actuators 28 and the electrically operated valves 60 controlling them, the position of the reversing switch means 68 determining the operation of the valves 60 in direction corresponding to whether the actuators are being extended or contracted.

A pump 71 pumps the hydraulic fluid to the actuators, the pump having connection with a reservoir or sump 72 and a feedline 74 leading through the valve 70, through branches 78, the valves 60, and to the actuators 28 on the side thereof for extending them. The

fluid is directed for extending or contracting movements in a manner described below. Also shown is a return line 75, for the return of hydraulic fluid from the actuators, through valve 70 back to the pump 72.

FIG. 6, as in the case of FIG. 5, includes the operating and control components for the same two actuators of FIGS. 1, 3, 4, and the additional ones for the other two of the four actuators of FIG. 2, the latter being those to the left and below the dot-dash line 77 of FIG.

6. The same can be said with respect to FIG. 7, those to the right of the dot-dash line 79 being of the two-actuator arrangement while the whole diagram is of the fouractuator arrangement.

FIG. 7 shows the complete hydraulic circuit, including the pump 71 and the valve 70, the latter being of known type, having a neutral position represented in FIG. 7 in which there is no flow through the valve, and each of opposite positions in which there is flow through the valve in respectively opposite directions.

The circuit of FIG. 7 in addition to the line 78 has another line 82 having branches 82a, 82b, 82c, 82d leading to the actuators on the side for contracting them. The valve means 60 includes electrical operating means 84, in the form of solenoids for actuating the valves proper. The check valve means 80 are arranged for permitting flow of fluid into the actuators on the side for extending them, and opened for flow of fluid in the opposite direction by pressure in the line 82 through lines 86. In one phase of operation, as the hydraulic fluid flows through the line 78 and the valves 80 readily open, while the valves prevent flow of the hydraulic fluid in the opposite direction unless opened by the pressure established in the line 82 and the lines 86. Whether pressure is established in the lines 86, is determined by the position of the valve 70 as described hereinbelow.

For extending the actuators and lifting the load, the valve 70 is manually shifted to UP position (FIG. 7), and in this position the fluid flows through the lines and branches 78, through the check valves 80 to the piston side of the actuators. For contracting them, the valve 70 is moved to DOWN position, in which the fluid flows through the lines 82 to the rod side of the actuators, the pressure in the lines 86 opening the valves 80 enabling the fluid from the piston side to exit. In both of these situations, the valves 60 remain in open position (as in FIG. 7) and so long as the actuators all move at the same rate, within the permissible range of variation, the valves 60 remain in open position, and only move to or toward closed position when such variation exceeds that range.

In the case of variation in speed of movement of the actuators, assume that the slave actuator 28b should move faster than the master actuator 28a (FIGS. 5, 6, 7 the disc 46b would advance to bring the finger 48b into engagement with the corresponding contact element 42 (FIG. 6) and energize the solenoid 84b (FIG. 7) to shift the valve 60b to or toward closed position, blocking the flow of fluid through that valve, either to or from the actuator 28b, momentarily stopping or slowing the movement of the piston 32b, as a result of which the finger 48b separates from the contact element 42 and the valve 60b moves back to its open position, and flow resumes to the actuator (28b).

The manual shifting of the valve 70 also shifts the switch means 68 to a corresponding position; e.g., in the extension movements of the actuators, the switch means, being in a corresponding position, effect shutting off the flow of fluid to the piston side of the particular actuator, which extends too fast, while in the contracting movements, the switch means are in the opposite position and in the same manner shut off flow of fluid from the piston side of the particular actuator, which contracts too fast.

The invention is also adaptable to many refinements, one of which is represented in FIG. 8, in which the cable 45 as in FIG. 3, is shown driven by the pinion 36b, but instead of being connected at its driving end to the disc 46 (FIG. 3) it is connected with a gear in mesh with a smaller gear 92 driving the disc 46. Correspondingly the actuator 28a through its rack 34 drives the pinion 36a and therewith gear 94 in mesh with gear 95, driving the disc 40. In this arrangement, with identical gears 90 and 94 larger than the identical gears 92 and 95, the ordinary movements of the actuators 28 are multiplied for moving the discs 40, 46 at a faster rate, whereby to provide a more sensitive response and finer control.

FIG. 9 represents the application of the four-actuator arrangement to steering a vehicle in which four wheels are steered. In this figure the vehicle 96 has individually steered wheels 98 the right rear one of which, 98a, may be considered the master. This wheel 98a has a gear 100a oscillated according to the steering of the wheel, which is effected under suitable and known manual control, and it meshes with a gear 102a driving a cable 45 to the gear 62 identified in FIG. 5. In this embodiment of the invention the front and rear wheels are arranged for steering in either direction, in a tight turn, the sensing elements moving respectively in the same directions.

FIG. 10 shows a vehicle in which the invention is applied in the form of the Z-actuator arrangement. In the present arrangement the vehicle includes front wheels 104 arranged for steering together about a common axis 105, which upon turning rotate a gear 106 in mesh with a gear 108 driving the cable 45 and the disc 40 identified in connection with FIG. 3. The rear wheels 110 upon steering on a common axis 111 rotate a gear 112 in mesh with a gear 114 which through the cable 47 drives the disc 46, with control effective thereon as described above. The degree of steering assist by the rear axle wheels in relation to the front axle wheels can be varied to any desirable value and is dependent on the respective gear ratios 106 over 108 and 112 over 1 14.

FIG. 11 represents the application of the invention to a device such as a lift truckhaving fork members 116 having the same racks 34 driving the pinions 36, which acting through the cables 45 drive the discs 40, 46 with corresponding control. With the provision of a known mechanical or known electro-mechanical override device in either the discs 40, 46 or the pinions 36, the fork members can be pre-positioned to an irregular shaped load, after which the invention is applied to provide uniform lifting of said load.

FIG. 12 shows the invention applied to airplane elements such for example as trimtabs 118. The trimtabs are driven by actuators 120 for swinging them about an axis 122, and each trimtab is provided with an arm I24 driving a rack 34 which in turn rotates a pinion 36. The pinions are drivingly connected through the cables 45 with corresponding discs 40, 46, as described above.

FIG. 13 shows the invention applied to a power device operating in a horizontal direction, as contradistinguished from a lift device. The operation is precisely the same as described above in connection with the other phases of the invention, the only difference being that the force is applied in horizontal direction. in the present instance the actuators 28 are utilized for pushing an instrumentality such as a handling ring 130 utilized for a known purpose. The actuators 28 include the pistons 32 identified above on which are mounted the racks 34 meshing with pinions 36, and at one end of the cable 45 is a unit 132 which contains the discs 40, 46 (FIG. 3) and pertinent elements.

The device is also adapted to electrical control, as

v represented in FIG. 14, which includes rollers 132 driven through belts 134 by variable speed electric motors 136. The control contact elements 42, 48

identified above are provided respectively on the rolactuators, manually shiftable valve means for controlling the direction of flow of fluid to the actuators for correspondingly extending and contracting them, sensing means including a member driven by each of the slave actuators, and further including a plurality of members driven by the master actuator and including one corresponding to and forming a pair with each of the members associated with the slave actuators, the sensing members being moved at a rate proportional to the rate of movement of the actuators, the sensing members as between the master actuator and each slave actuator including two spaced electrical contacts forming sensing elements on a first of the members and a single electrical contact forming a sensing element on the other member and positioned between the two elements on the first, an electrically operated valve controlling the rate of flow of fluid to or from each actuator, an electrical circuit including said valves and also including a common conductor to the valve of the master actuator, reversing switch means individual to the slave actuators connecting the pair of sensing elements related thereto, the switch means in one position connecting the valve of the master actuator to one of said two sensing elements of each pair of members and connecting the valves of the slave actuators individually to the other sensing element of respective pairs of members, and in the other position, reversibly connecting the valves of the master and slave actuators to the sensing elements, and means responsive to the shifting of the manually shiftable valve for shifting the switch means between their opposite positions, said sensing means being arranged so that upon differential in movement of the sensing members of each pair, the one sensing element on said other member contacts one or the other of the two sensing elements on the first member according to increase or decrease in rate of movement, and effects control of said valves according to the direction of movement of the actuators.

2. A power device according to claim 1 wherein the electrical circuit includes a common conductor connected to all said single sensing elements, and a common conductor connected to the valve of the master actuator and the reversing switch means and through the latter to a selective one of said two sensing elements of each of said first members.

3. A power device according to claim 1 and including check valves individualto the actuators enabling free flow of fluid therethrough and into the actuators in extending direction of. the actuators, and operative when closed for checking flow of fluid in the opposite direction, and hydraulic means in the circuit for providing pressure for opening said check valves in the normal flow of fluid in said opposite direction, and wherein the electrically operated valves are operative in an open position for enabling flow of fluid in both directions to and from their respective actuators, and in a closed position operative for blocking flow in both directions to and from the actuators.

4. A power device comprising a plurality of movable actuators including a master actuator and a slave actuator spaced apart a substantial distance,

power means,

means individual to the actuators for transmitting power from the power means to them,

control means individual to the power transmitting means, sensing means including a pair of juxtaposed, co-

axial rotary discs, a first of the discs having a pair of angularly spaced first control elements facing the second disc and the second disc having a second control element positioned between the first control element and operative for engaging one or the other of the first control elements corresponding to difference in rate of rotation of the discs, means for rotating the discs respectively by the actuators and at a rate according to the rate of movement of the actuators,

the disc rotating means including, in the case of at least one of the discs, a flexible cable torsionally rotated by the respective actuator and the cable being operative, acting by means of the torsion imparted thereto, for rotating the associated disc, and

control means operative in response to engagement of the said second control element with either of the said first control elements for varying the speed of movement of the slave actuator and thereby synchronizing the speed of movement of the actuators.

5. A power device according to claim 4 wherein the actuators are arranged with members movable linearly, and rack and pinion means is operatively interposed between each said movable member and the corresponding disc for rotating the disc.

6. A power device according to claim 5 wherein the actuators are arranged with their said members movable vertically for lifting and lowering loads.

7. A power device according to claim 5 wherein the actuators are arranged with their said members moving longitudinally for pushing a load.

8. A power device comprising a plurality of movable actuators including a master actuator and a plurality of slave actuators,

power means,

means individual to the actuators for transmitting power thereto for moving them,

control means individual to the power transmitting means, sensing means operatively connected with the master actuator and including a member for each of the slave actuators, and the sensing means further including a sensing member operatively associated with each slave actuator, the members associated with the slave actuators and the corresponding members associated with the master actuator having interfitting control elements, the sensing means members being moved by, and at a rate in proportion to the rate of movement of, the respective actuators, and when the members move at a common uniform rate the interfitting control elements remain out of interengagement, and

means operative in response to interengagement of the control elements on the members associated with the master actuator and any of those on any of the members associated with the slave actuators, due to variation in rate of movement of the actuators for controlling the associated control means for thereby synchronizing the speed of the actuators.

9. A power device according to claim and including means for rotating the discs at a rate different from that of the pinions of the rack and pinion means whereby to provide a different sensitivity to control provided by the same rate of rotation.

10. A power device according to claim 9 wherein the discs are rotated at a rate greater than that of the pinions whereby to provide greater sensitivity to control than provided by the same rate of rotation.

11. A power device according to claim 5 wherein the actuators are incorporated in an instrumentality which includes elements each swingably movable about an axis, the actuators are arranged for rectilinear movement for so moving said elements, and the rack of the rack and pinion means is moved rectilinearly by said elements, and thepinions thereof are rotated by the rack.

12. A power device according to claim 8 wherein the sensing means members are .rotatable discs including one associated with and driven by each of the actuators, and the related discs as between the master actuator and the slave actuators include a pair of sensing elements on one of the discs spaced apart circumferentially, and a single sensing element on the other disc sensing elements thus related being positioned for interengagement upon variation in rate of movement between the master actuator and any of the slave actuators.

13. A power device according to claim 8 in which the sensing means associated with the master actuator includes a first disc driven directly by the master actuator and a first gear connected for rotation therewith, and an additional gear in mesh with the first gear and driven at the same rate thereof, and additional 'discs connected for rotation with said second gear and thus for rotation at the same rate as the first disc, and the sensing means also includes a disc driven by each of the slave actuators and associated with corresponding ones of the discs driven by the master actuator. 

1. A power device comprising a plurality of double acting hydraulic actuators including a master actuator and a plurality of slave actuators, means including a hydraulic circuit for transmitting fluid to or from the actuators, manually shiftable valve means for controlling the direction of flow of fluid to the actuators for correspondingly extending and contracting them, sensing means including a member driven by each of the slave actuators, and further including a plurality of members driven by the master actuator and including one corresponding to and forming a pair with each of the members associated with the slave actuators, the sensing members being moved at a rate proportional to the rate of movement of the actuators, the sensing members as between the master actuator and each slave actuator including two spaced electrical contacts forming sensing elements on a first of the members and a single electrical contact forming a sensing element on the other member and positioned between the two elements on the first, an electrically operated valve controlling the rate of flow of fluid to or from each actuator, an electrical circuit including said valves and also including a common conductor to the valve of the master actuator, reversing switch means individuaL to the slave actuators connecting the pair of sensing elements related thereto, the switch means in one position connecting the valve of the master actuator to one of said two sensing elements of each pair of members and connecting the valves of the slave actuators individually to the other sensing element of respective pairs of members, and in the other position, reversibly connecting the valves of the master and slave actuators to the sensing elements, and means responsive to the shifting of the manually shiftable valve for shifting the switch means between their opposite positions, said sensing means being arranged so that upon differential in movement of the sensing members of each pair, the one sensing element on said other member contacts one or the other of the two sensing elements on the first member according to increase or decrease in rate of movement, and effects control of said valves according to the direction of movement of the actuators.
 2. A power device according to claim 1 wherein the electrical circuit includes a common conductor connected to all said single sensing elements, and a common conductor connected to the valve of the master actuator and the reversing switch means and through the latter to a selective one of said two sensing elements of each of said first members.
 3. A power device according to claim 1 and including check valves individual to the actuators enabling free flow of fluid therethrough and into the actuators in extending direction of the actuators, and operative when closed for checking flow of fluid in the opposite direction, and hydraulic means in the circuit for providing pressure for opening said check valves in the normal flow of fluid in said opposite direction, and wherein the electrically operated valves are operative in an open position for enabling flow of fluid in both directions to and from their respective actuators, and in a closed position operative for blocking flow in both directions to and from the actuators.
 4. A power device comprising a plurality of movable actuators including a master actuator and a slave actuator spaced apart a substantial distance, power means, means individual to the actuators for transmitting power from the power means to them, control means individual to the power transmitting means, sensing means including a pair of juxtaposed, co-axial rotary discs, a first of the discs having a pair of angularly spaced first control elements facing the second disc and the second disc having a second control element positioned between the first control element and operative for engaging one or the other of the first control elements corresponding to difference in rate of rotation of the discs, means for rotating the discs respectively by the actuators and at a rate according to the rate of movement of the actuators, the disc rotating means including, in the case of at least one of the discs, a flexible cable torsionally rotated by the respective actuator and the cable being operative, acting by means of the torsion imparted thereto, for rotating the associated disc, and control means operative in response to engagement of the said second control element with either of the said first control elements for varying the speed of movement of the slave actuator and thereby synchronizing the speed of movement of the actuators.
 5. A power device according to claim 4 wherein the actuators are arranged with members movable linearly, and rack and pinion means is operatively interposed between each said movable member and the corresponding disc for rotating the disc.
 6. A power device according to claim 5 wherein the actuators are arranged with their said members movable vertically for lifting and lowering loads.
 7. A power device according to claim 5 wherein the actuators are arranged with their said members moving longitudinally for pushing a load.
 8. A power device comprising a plurality of movable actuators including a master actuator and a plurality of slave actuators, power means, means individual to the actuators for transmitting power thereto for moving them, control means individual to the power transmitting means, sensing means operatively connected with the master actuator and including a member for each of the slave actuators, and the sensing means further including a sensing member operatively associated with each slave actuator, the members associated with the slave actuators and the corresponding members associated with the master actuator having interfitting control elements, the sensing means members being moved by, and at a rate in proportion to the rate of movement of, the respective actuators, and when the members move at a common uniform rate the interfitting control elements remain out of interengagement, and means operative in response to interengagement of the control elements on the members associated with the master actuator and any of those on any of the members associated with the slave actuators, due to variation in rate of movement of the actuators for controlling the associated control means for thereby synchronizing the speed of the actuators.
 9. A power device according to claim 5 and including means for rotating the discs at a rate different from that of the pinions of the rack and pinion means whereby to provide a different sensitivity to control provided by the same rate of rotation.
 10. A power device according to claim 9 wherein the discs are rotated at a rate greater than that of the pinions whereby to provide greater sensitivity to control than provided by the same rate of rotation.
 11. A power device according to claim 5 wherein the actuators are incorporated in an instrumentality which includes elements each swingably movable about an axis, the actuators are arranged for rectilinear movement for so moving said elements, and the rack of the rack and pinion means is moved rectilinearly by said elements, and the pinions thereof are rotated by the rack.
 12. A power device according to claim 8 wherein the sensing means members are rotatable discs including one associated with and driven by each of the actuators, and the related discs as between the master actuator and the slave actuators include a pair of sensing elements on one of the discs spaced apart circumferentially, and a single sensing element on the other disc, sensing elements thus related being positioned for interengagement upon variation in rate of movement between the master actuator and any of the slave actuators.
 13. A power device according to claim 8 in which the sensing means associated with the master actuator includes a first disc driven directly by the master actuator and a first gear connected for rotation therewith, and an additional gear in mesh with the first gear and driven at the same rate thereof, and additional discs connected for rotation with said second gear and thus for rotation at the same rate as the first disc, and the sensing means also includes a disc driven by each of the slave actuators and associated with corresponding ones of the discs driven by the master actuator. 